HMS Wilton — a glass-reinforced plastics minehunter

HMS Wilton- a glassreinforced plastics minehunter J. A. G. THOMAS*

On 18 January HMS Wilton, a 46-metre minehunter, built of glass-reinforced plastics, was launched. This is reputed to be the largest GRP ship in the world and its design and construction represent a milestone in the history of shipbuilding and the technology of composite materials.

BA CKG R 0 UND Coastal minesweepers for the UK Ministry of Defence (Navy) have traditionally been built of wood on an aluminium alloy structural framework. Magnetic materials have to be avoided as far as possible to lessen the risk of exploding magnetic mines. Nearly 100 ships of this class, all given place names ending in - t o n , were built and they are now nearing the end of their useful lives. The Ministry of Defence has been, for some years, evaluating glass-reinforced plastics for various marine uses and as an alternative material for the next series of mines countermeasures vessels. As a part of this programme, two midship sections of the - t o n class of ship were moulded in GRP at the Vosper Thornycroft yard at Woolston and compared with a glued laminated timber test section build by Vosper Thornycroft at their Portchester yard. One of the GRP sections was of sandwich construction and was built in collaboration with Bristol Aeroplane Plastics Limited, now Rolls-Royce (Composite Materials) Limited, and the other was a solid laminate. Tests were carried out at the Naval Construction Research Establishment at Dunfermline, Scotland and on the basis of these, the solid GRP laminate was selected. The sandwich construction proved to have inadequate resistance to the high shock forces which would arise from nearby underwater explosions. In February 1970 the Ministry of Defence (Navy) ordered a new ship based as closely as possible on the earlier -ton class but in GRP. HMS Wilton, as the ship was named, was launched on 18 January 1972.

DESIGN HMS Wilton is not intended to be the first in a line of identical ships. Rather she is a prototype for the evaluation of a new shipbuilding material, GRP, and new methods of construction. This is why her design is a close copy of the existing - t o n class of coastal minesweepers (see Table 1). The performance and characteristics of these ships is well known and comparisons of HMS Wilton with other ships of the same class will give information about the performance of GRP. *

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Composites

Most of the equipment, including main engines, fitted to HMS Wilton, has been transferred from HMS Derriton.

~VIATERIA LS The resin used was a non-pigmented isophthalic polyester with a high heat distortion temperature, low water absorption and good delayed lay-up characteristics. It was specially formulated for this application by BP Chemicals, Barry, South Wales, and is based on their Cellobond resins. A similar resin has been used successfully over the last few years in the construction of the casing and fins of Oberon class submarines. Nearly all the glass fibre reinforcement used was Tyglas Y920 woven roving made by Fothergill & Harvey Limited from Owens-Coming Fiberglas Araton-sized roving. In all, about 84 000 square metres of glass cloth (70 miles of woven rovings) have been used. During mechanized lay-up,

Table I Principal dimensions and details of HMS Wilton

Length overall Beam Depth to upper deck Displacement (full load) Engines

Maximum speed

46 m 8-5 m 4-5 m 453 tonnes twin English Electric Deltic diesels, 1500 bhp each 16 knots

the good drape characteristics of the glass cloth enable it to conform to hull contours without distortion or wrinkling. It can be manufactured in long lengths to a consistent, fault-free standard. Table 2 shows values for mechanical properties of test specimens of GRP laminate taken from the ship during construction. The value of ultimate tensile strength used for design was conservative and lower than the experimental figure given in Table 3, which compares tensile strength of this GRP with other materials. Since the GRP is of laminar construction, the bond between the layers is less strong than the bulk material and this has been allowed for in the design. The resulting GRP hull is stronger than one in steel, without a prohibitive increase in weight.

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The GRP has been found to show negligible loss of strength on prolonged immersion in sea water under pressure. There is no loss in strength at temperatures up to extreme tropical conditions and at Arctic temperatures the strength increases with no embrittlement of the material. Repairs to damaged sections carried out using the same glass and resin gave strengths of more than 90% of the original.

FIG 1 HMS Wilton, a 46-m GRP minehunter, just before being launched bows first at the Woolston, Southampton shipyard of Vosper Thornycroft Limited Table 2 Mechanical properties of GRP laminate used in HMS Wilton. These values are based on tests and were used in the design calculations

(Units 227 15.5 x 206 13.7 x 185 13-7 x 110 3.43 x 13.7

Ultimate tensile stress (UTS) Tensile modulus (Young's modulus) Flexural strength Flexural modulus Compressive strength Compressive modulus Shear strength Shear modulus Interlaminar shear strength

MN/m2) 103 103 103

Thermal insulation inside the GRP hull has been found to be nearly as good as for wood and better than for steel hulls. Tests for fire resistance have shown that the load-bearing characteristics of this type of GRP in fire were better than those of aluminium alloy. The surface layer of glass cloth provides a barrier which greatly reduces the rate of propagation of fire. In toughness, the GRP is superior to both mild steel and aluminium alloy. Crack propagation is resisted by the alternate strong layers of glass and weak layers of resin. Fatigue tests on specimens in water indicated a fatigue limit of about 25% of the ultimate tensile strength. The GRP is, of course, less stiff than traditional shipbuilding materials; the tensile modulus is about 1/13th that of steel and one quarter that of aluminium. Vosper Thornycroft have overcome this lack of stiffness by good design and by using greater thicknesses of GRP than would be used for heavier steel, especially in relation to compressive load and the positioning of machinery and equipment. The greater resilience of GRP gives it more resistance to shock loading, a useful attribute for a minehunter. A creep limit of 40% ultimate tensile strength has been established for this type of GRP. Therefore for all parts of the ship subject to alternating loads, which is most of it, fatigue failure will occur before creep failure. Chemical resistance is good and integral fuel tanks (and fresh water tanks) are used, the material being unaffected by oils and fuels.

STRUCTURE The solid laminate structure consists of a single skin of GRP, in general about 32 mm thick, with transverse framing, bulkheads and deck panels bonded-in. Some longitudinal stiffeners, including engine bearers, are also worked in, but these are kept to a minimum because the laminating of their intersections with transverse frames is a complex and time-consuming task. Transverse framing was also the scheme for the wood and aluminium coastal minesweepers. The overall scheme was selected as giving the greatest resistance to the shock loads imposed when an underwater explosion takes place near the hull.

103

Table 3 Comparison of tensile strength of the type of GRP laminate used in HMS Wilton with other materials Material

Specific gravity

UTS (MN/m 2)

Specific UTS (MN/m 2)

Polyester/chopped strand mat GRP*

1 ~,

103

Polyester/woven roving GRP T

1-6

276

172

345

203

73.6

Polvester/woven cloth GRP

1-7

Mild steel

7-8

Aluminium alloy N8

2.7

276

102

Pine (along grain) (across grain)

0-5

76 6

152 12

434 (241 yield)

55.6 (30-9)

* This is the type of GRP commonly used commercially in small craft construction t As used for HMS Wilton

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FIG 2 Hull structure nearing completion, with decks still to be fitted. Transverse framing is clearly visible

FIG 3 Deck panel with beams, longitudinals and knee~ The laminate has not yet been laid-up over the po~urethane foam block cores for the two inboard longitudinals Frames are constructed by placing formers cut from blocks of rigid polyurethane foam in position and laying-up laminate over them to give a 'top-hat' section (see Fig 4). The flanges are tapered off where they are bonded to the hull shell to avoid abrupt changes in section. Although this type of bonding is entirely satisfactory for all normal structural purposes, including shipbuilding, underwater explosions can bring about failure of the flange-to-shell bond in direct tension. Therefore in HMS Wilton, these joints are reinforced by through-bolts of a special bronze. Below the waterline, where the shell is thicker, the bolt heads are countersunk to give a flush finish which will not create resistance by disturbing the water flow. In the thinner topsides countersinking is not possible and raisedhead bolts have been used; these are visible on the finished hull (see Fig 6). Bulkheads, lower deck panels and partitions are bonded into the structure in much the same way as frames, with reinforcing knees at junctions. The whole of the structure, including fuel and water tanks, sonar housing, and many small partitions and sub-assemblies are of GRP laminate bonded-in.

The hull was laid-up in a female steel mould by a mechanized process. The mould is built in sections which are bolted together with the framing outside. Dimensional tolerances were ---3 mm and a high degree of fairness was attained (see Fig 5). A separate moulding was made for the forefoot - the narrow lower part of the stern, which was bonded and bolted onto the main hull moulding afterwards. This helped to ensure good access for laminating in this rather confined space and also helped with ventilation and sweeping out during construction. The lay-up of the hull was conducted as a mainly mechanized process, which consisted or resin and woven glass dispensers mounted on a gantry and moving transversely: the gantry moves along inside the hull mould. The resin constituents are automatically blended in their correct proportions and the glass cloth impregnated with them to a 50/50 resin/glass ratio by weight before the impregnated cloth is fed into the mould transversely in continuous lengths. The only manual assistance needed is for workers on movable platforms mounted on the gantry to consolidate the laminate with long-handled rollers. These dispensers which were specially designed by Vosper Thornycroft proved largely successful in laying-up 100 tons of glass reinforcement and a somewhat greater quantity of resin. However, the more complex moulding tasks, such as the laying-up of frames, beams, longitudinals

Light foorn

GRP

.

FIG 4 Diagrammatic section o f 'top-hat' stiffener as used for beams and longitudinals

FA BRICA TION The maximum length of vessels built prior to HMS Wilton has been about 26 m. The greatest increase in the size of Wilton has raised a number of problems not previously encountered. Special facilities are required to provide the controlled, warm, dry conditions necessary for consistently good lay-up of GRP. Minimum laminate temperatures of 15°C are required. Also, ventilation and fire precautions during layup must be good. The styrene content in the atmosphere must always be at less than 100 parts per million. Flameproof electrical equipment is fitted and only cleaning fluids with high flash points are used. Smoking is banned and fire doors are fitted every 15 m round the walls, Fire detectors in the roof are linked directly to the local municipal fire stations.

FIG 5 Hull mould. The external framing and the aperture for the separate forefoot moulding can be seen. White lines on the black mould surface show through the translucent laminate and help to locate reinforcement cloths. The fairness o f the mould surface is obvious

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first, and tapered off forwards over the number of layers so that none was too fully cured when the next was applied. The assembly of decks and beams in the hull, with laminated knees connecting beams to frames, was carried out towards the end of the programme, followed finally by the superstructure and bridge.

THE FUTURE

FIG 6 H M S Wilton nears completion with superstructure and bridge f i t t e d

and their intersections, were not entirely practical with these dispensers, and much of this work had to be done by hand. Developments are envisaged in the design of dispensers to make them more suitable for this type of work.

The Royal Navy look on HMS Wilton as a purely experimental prototype and not the first of a new class of mines countermeasures vessels. A new design of vessel, which will be bigger than the present coastal minesweepers, is needed and the material and type of construction selected for this vessel must depend on the results of sea trials with HMS Wilton. At a project cost of one and a half to two million pounds HMS Wilton is an expensive ship, but Vosper Thornycroft claim the following economic advantages for GRP ships: (1)

(2)

Q U A L I T Y CONTROL Mechanized lay-up controlling the resin/glass ratio reduces dependence on the operator. Regular checks were made on the base resin and glass during fabrication and the lay-up of each lamination was inspected and logged and pieces of finished laminate mechanically tested. The use of a clear unpigmented resin gave a translucent material in which any flaws could be seen. The presence of pigments can also adversely affect laminating properties. During lay-up, the building was kept scrupulously clean to minimize dirt inclusion in the laminate, which can create points of weakness. A rapid rise in temperature can occur in the laminate when the resin is curing. Therefore it was found safest to lay-up a maximum of 12 freshly impregnated piles during any 12 hour period. For good bonding between individual laminae, the top lamina should not be fully cured when the next is laid-up. Satisfactory bond strengths are maintained with delays of up to seven days between laminations. For longer delays, a GRP ply is used to cover the top lamina and this is removed before laminating is continued. Should an unscheduled delay occur, the surface can be prepared by surface abrasion or chemical priming but these methods are not ideal as a general technique.

ORGANIZATION Much more detailed planning was needed than for a conventional ship of this size, because of the time limits imposed by the materials. Once the constituents of the resin system are mixed, the time available for lamination is limited. Also, the longer one lamina is allowed to cure the less good is the bond between it and the next lamina. To minimize these problems, a fairly slow curing resin was selected and moulding of the main hull was carried out progressively "from stern to bow, with the skin laminate built-up so that it reached its full thickness next to the transom (a separate moulding bonded on subsequently)

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(3) (4) (5)

The price of glass fibre and resin are stable, whereas the cost of steel has risen rapidly with inflation. GRP ships have low maintenance costs. Exactly how low, HMS Wilton will tell us, but Vosper Thornycroft believe that the cost of a new hull could probably be saved over 15 years. Low maintenance means less time in port. Labour costs can be reduced by improving production techniques. Improved design can lead to the minimum amount of material being used to achieve necessary properties.

A preliminary study by Vosper Thornycroft suggests that a 43-m light warship employing an improved type of GRP sandwich construction could be an economic proposition. With the present state of knowledge, Vosper Thornycroft feel that 60 m is probably the maximum length feasible for a ship of this type, but with more design experience longer ships should be possible. There will be no repeats of Wilton and the Navy will see how their prototype behaves before ordering any other GRP ships, and this will not be before 1973. However, Vosper Thornycroft seem confident that their expensive facilities will be fully utilized and they admit to one or two enquiries from abroad. Two such facilities operating sideby-side would make most economic sense, since a cycle of lamination in one and dismantling of the mould in the other could then be operated. At present costs, GRP is only likely to be feasible for large ships requiring a special property. Being non-magnetic is one such property. Another could be chemical resistance which would be important for carrying corrosive liquids; or insulation, which would be necessary for the carrying of cold gases, such as liquified natural gases, which cause embrittlement of steel.

REFERENCES Evaluation studies and the building programme of HMS Wilton have been covered in the following articles: 1 Composites, Vol 1, No 1, pp 52, 53 (September 1969) 2 Composites, Vol 2, No 1, p 5 (March 1971)

3 Beale, R. F., British Polymer Journal, Vol 3, No 1 (January 1971) 4 Gibbs, H., 'Materials for Marine Structures', a paper presented at the 4th International TNO Conference, 24-26 February 1971, Utrecht